(1) Field of the Invention
The present invention relates to solid conductive electrolyte compositions incorporating a branched polyhydroxyalkanoate (PHA) polymer and a salt of a conductive metal and to a method for producing such compositions. In particular, the present invention relates to the use of polyhydroxyalkanoates (PHA) in such compositions which are polymers occurring in nature and thus biodegradable.
(2) Prior Art
Polymeric electrolytes are of interest as alternatives to liquid for use in batteries and other applications because they do not leak, they tolerate volume changes, and their malleability permits great flexibility in battery design (MacCallum, J. R. and Vincent, C. A., eds Polymer Electrolyte Reviews--1, Elsevier Applied Science, N.Y., pp. 1-21 (1987); and MaoCallum, J. R. and Vincent, C. A., eds Polymer Electrolyte Reviews -2, Elsevier Applied Science, N.Y., pp. 23-37 (1989)). The most widely studied materials for this purpose have been polyethers, in particular poly(ethylene oxide) (PEO) and poly(1,2-propylene oxide) (PPO).
Conductivity in ionic polymers is restricted to the amorphous phase above the glass transition temperature (Tg) (Gray, F. M. "Solid polymer electrolytes" VCH, pp 1-33 (1992)), because, although the polymer solvent is immobile in the macroscopic sense, ionic conductance results from local motions of the polymer segments. Polymers suitable for the preparation of solid electrolytes should have the following features: 1) atoms or groups of atoms containing electron pairs with a donor power strong enough to coordinate a metal cation; 2) a suitable distance between the coordinating centers to optimize the formation of multiple intrapolymer ion bonds with cations; and 3) low barriers to bond rotation to facilitate segmental motion of the polymer chain. Good mechanical properties such as malleability are also important.
In the case of polyethers, polyethylene oxide --(C--C--O)-- has an optimal backbone. Polyethers in which the distance between oxygens in the backbone is less, e.g. polymethylene oxide --(C--O)-- or greater e.g. 1,3 polypropylene oxide --(C--C--C))-- do not significantly dissolve salts. Changes in the pendant substituents e.g. 1,2-polypropylene oxide --(C(C)--CO)-- form polymer electrolytes with somewhat poorer conductance because of the stearic hindrance introduced by the methyl substituent (Cowie, J. M. G. In "Polymer electrolyte reviews-I" (eds. J. R. MacCallum and C. A. Vincent) Elsevier Applied Science, New York, N.Y., pp 69-101 (1987); and MacCallum, J. R. and C. A. Vincent. In "Polymer electrolyte reviews-I" (eds. J. R. MacCallum and C. A. Vincent) Elsevier Applied Science, New York, N.Y. pp 23-37 (1987)).
Synthetically produced, non-branched polyesters may also form conducting salt complexes. Poly-.beta.-propiolactone (Cowie, J. M. G. In "Polymer electrolyte reviews-I" (eds. J. R. MacCallum and C. A. Vincent) Elsevier Applied Science, New York, N.Y., pp 69-101 (1987)) complexes with lithium perchlorate had conductivity rivaling that of the polyethers (Watanabe, M., M. Togo, K. Sanui, N. Ogata, T. Kobayashi, and Z. Ohtaki. Macromolecules 17:2908-2912 (1984)). There has been little commercial interest in polyesters becaus they are poorer electron donors and they are more difficult and expensive to synthesize. The backbone of PPL --(O--C--C--CO)-- is identical to that of the poly-.beta.-hydroxyalkanoates (PHA) --(O--C(C)--C--CO)-- which are a family of natural polyesters produced by microorganisms. Naturally occurring PHAs are optically active (R) polyesters that are best known as intracellular reserves in bacteria (Anderson, A. J. and Dawes, E. A., Microbiol. Rev. 54:450-472 (1990)). Microorganisms are capable of producing a wide range of polymers and copolymers based on 3-hydroxypropionic acid substituted with various lower alkyl groups in the 3-position (or .beta.-position) which can be used to prepare polymer electrolytes with different mechanical and thermal properties. The most common homopolymer is poly-3-hydroxybutyrate (PHB) which has a methyl group in the 3-position, but polymers containing C2 to C10 alkyl side groups, alkenyl side groups, and 4-hydroxy acids can also be produced (Holmes, P.A., "Developments in Crystalline Polymers - 2" (D. C. Bassett, ed) Elsevier Applied Science, N.Y., pp 1-65 (1988); Anderson, A. J. and Dawes, E. A., Microbiol. Rev. 54:450-472 (1990); and Lageveen, R. G., Huisman, G. W., Preusting, H., Ketelaar, P., Eggink, G., and Witholt, B., Appl. Environ. Microbiol. 54:2924-2932 (1988); Marchessault, R. H. and C. J. Monasterios. In "Biotechnology and polymers" (ed. C. G. Gebelein) Plenum Press, New York pp 47-52 (1991)). Some organisms, e.g. Pseudomonas oleovorans, are able to accumulate PHAs with longer side-chains including unsaturated ones when the appropriate substrate is added to the culture medium (Preusting H., A. Nijenhuis, and B. Witholt. Macromolecules 23:4220-4224 (1990)). The homopolymers and copolymers are all biodegradable. Their rate of chemical hydrolysis at neutral pH is extremely slow, but microorganisms produce both specific and non-specific enzymes capable of degrading the polymers rapidly to non-toxic monomers. The monomers are all optically active in the R absolute configuration. PHB can be produced by the microorganisms from carbon substrates as diverse as glucose, ethanol, acetate, alkanes, alkenes, methane and even gaseous mixtures of carbon dioxide and hydrogen. The polymer exists as discrete granules within the cell cytoplasmic space and can represent up to 80% of the dry cell weight. After extraction and purification, PHB behaves as a normal crystalline thermoplastic with a melting point around 180.degree. C. (the other PHAs have lower melting points --down to 50.degree. C.) and it can be processed by conventional extrusion and molding equipment.
These naturally occurring polyesters have been exploited commercially as biodegradable thermoplastics. There is no mention of the use of these polymers as electrolytes in the PHA literature (Holmes, P.A. In "Developments in Crystalline Polymers --2" (D. C. Bassett, ed) Elsevier Applied Science, New York, pp 1-65 (1988)) or in the literature on polymer electrolytes.
Industrial processes have been developed which will make the PHAs at competitive cost. At present, PHB and the other naturally produced polymers and copolymers produced in some bacteria and archaebacteria are the best sources of the polymers. The genes encoding PHA synthesis can be transferred to other organisms and PHAs can then be produced in the recipients (Slater, S. et al, Applied and Environmental Microbiol. 58, 1089-1094 (1992) and Poirier, Y. et al, Science 256 520-522 (1992)). Synthetic routes to the branched PHA's are difficult and expensive. The natural polymers can be mixtures of branched chain polymers which can make them more amorphus. The mixture is dictated by the growth medium used to feed the microorganisms. Generally the Tg to Tm range is greater for mixtures.